1. Field of the Invention
The present invention relates to the manufacture of chemically sensitive, mechanically stable low unsaturation elastomer, such as butyl rubber or EPDM latices which are easily manufactured to give commercial products of roughly 50-68% solids content and having a desirable low viscosity. These are generally prepared by emulsifying butyl rubber or EPDM in the form of hydrocarbon solutions using water and a surfactant. Butyl rubber latices constitute the preferred embodiment of the present invention.
2. Description of the Prior Art
Butyl rubber is, of course, a conventional widely used article of commerce today. Briefly, it is the copolymer of isobutylene or another isoolefin of closely related molecular weight with a diolefin, usually isoprene or butadiene, in which the major component is isobutylene and the minor component is the isoprene or butadiene. It is prepared by a copolymerization at low temperatures, i.e., of the order of -40.degree. to -164.degree. C. in the presence of a Friedel-Crafts type catalyst using, for example, 1 to 10 parts by weight of isoprene and 99 to 90 parts by weight of isobutylene. The polymerization may be stopped after the desired degree of polymerization has occurred through the introduction of an inactivator such as methyl alcohol, water, an amine, or isopropyl alcohol, after which the mixture is treated to recover the polymer from a water suspension by any convenient manner such as straining or filtering or otherwise as may be convenient. The polymer is then dried either as a blanket passing through a tunnel drier or on a mill. In connection with the present invention, the butyl rubbers employed in making up the latices generally have number average molecular weights of about 200,000 but other molecular weight butyl rubbers of 100,000 to 350,000 may also be employed.
The term "EPDM" is used herein in the sense of its definition as found in ASTM-D-1418-64 and is intended to mean an elastomeric terpolymer containing ethylene and propylene in the backbone and a diene in the side chain. Illustrative methods for producing these terpolymers are found in U.S. Pat. No. 3,280,082, British Patent 1,030,289 and French Patent 1,386,600. The preferred EPDM elastomers contain about 45 to about 80 weight percent ethylene, 2 to 10 weight percent diene monomer, the balance of the polymer being propylene. The diene monomer is preferably a nonconjugated diene and illustrative examples include hexadiene, dicyclopentadiene, ethylidene norbornene, methylene norbornene, propylidene norbornene and methyl tetrahydroindene. A typical EPDM polymer is one having a Mooney viscosity at 212.degree. F. of about 90 prepared from a monomer blend having an ethylene content of about 56 weight percent and a nonconjugated diene content of about 2.6 weight percent.
In preparing a butyl rubber or EPDM elastomer latex it is customary and conventional to dissolve the elastomer in a suitable solvent which is liquid under ordinary circumstances but which is later removed from association with the elastomer by distillation or other suitable readily accomplished means. Suitable solvents are usually the hydrocarbon solvents such as hexane, heptane, octane, isooctane, the higher fractions of paraffins such as Varsol, Solvesso 100 (a substantially 100% aromatic hydrocarbon fraction boiling between 315.degree. and 350.degree. F. ), benzene, and toluene. The solution of the elastomer in the conventional hydrocarbon solvents is termed a "cement". To this cement solution there is added an aqueous solution of emulsifier.
Often antifoaming agents are used in latex preparation to increase the rate of monomer or solvent stripping by preventing foam buildup. Oxyethylated alcohols or phenols, such as polyoxyethylated octyl or nonylphenol containing from 7 to 12 ethylene oxide units, higher alcohols such as 2-octanol, tributyl phosphate, milk, silicones, fatty acids and esters and many commercially available mixtures can be used for foam inhibition and foam breaking. Of course, these materials are used, if necessary, only in the amount required to achieve the desired results.
With the addition of the emulsifier and water to the butyl rubber or EPDM cement, the material is then subjected to an intense mixing operation wherein the two immiscible liquids are subjected to a suitable homogenization such as with a colloid mill, a sonic mixer, a dispersator, a Waring blender or the like. A particularly suitable homogenizer is the sonic mixer which consists of a pump which forces the material through an orifice and impinges the stream on a knife edge or a vibrating blade enclosed in a resonating bell. A suitable sonic homogenizer is the Sonolator which is marketed by Sonic Engineering Company. Additionally, the homogenizer manufactured by Gifford-Wood, or a colloid mill may be used or a combination of a homogenizer and a Sonolator may be employed in series. Generally, the emulsification is complete upon making three or four passes through the orifices and shearing surfaces. Once the emulsification or homogenization has been completed, the material which is called the raw emulsion or raw latex is next subjected to a stripping operation for the purpose of removing the hydrocarbon solvent and excess water still present in the emulsion. This is accomplished usually by heating the raw emulsion to from 100.degree. to 200.degree. F. or higher depending upon the particular solvent employed but at least sufficient to volatilize the solvent and to produce a finished latex. Vacuum can be used if desired. The hydrocarbon solvent and part of the water is removed by conventional means leaving the finished latex as a final mixture usually having a solids content ranging between about 50 and 68 weight percent of EPDM or butyl rubber. The latex particle size distribution generally has an average of less than one micron.
Various chemical classes of emulsifiers have been used in preparing latices. One particularly valuable class is the carboxylate soaps in which a carboxylic acid has reacted with an alkali metal hydroxide such as sodium hydroxide, potassium hydroxide, rubidium hydroxide, or cesium hydroxide, ammonia, or an amine such as methyl amine, ethyl amine or ethanolamine to form the soap. The carboxylic acid can be formed with an aliphatic hydrocarbon radical, either saturated or unsaturated, or a complex hydrocarbon ring system. Generally, in latex preparation, fatty acids such as oleic, stearic, palmitic, linoleic, naturally occurring mixtures of fatty acids, or the terpene and tall oil rosin acids have been used as emulsifiers. Soaps from these materials have the useful property of reacting with strong acids such as sulfuric, hydrocholoric and nitric, as well as organic acids such as acetic and formic acid; multivalent cations such as Ca.sup. .sup.+ , Al.sup.+ .sup.+.sup.+ , or Mg .sup.+.sup.+ ; salts such as cyclohexylammonium acetate; or ethyl alcohol, acetone, etc., so that their surface-activity is destroyed and the latex, prepared using these soaps, coagulates (coacervates). This is useful in coagulant dipping to form balloons, gloves, etc. and in the preparation of foam rubber and thread.
Butyl rubber latices have been prepared using alkali metal salts or ammonium salts of fatty acids such as potassium oleate or sodium oleate as the principal emulsifier. However, these latices generally require the use of other protective colloids and stabilizers to prepare them. In U.S. Pat. No. 2,944,038 it is taught that both polyvinyl alcohol and octylphenoxytriethoxyethanol are needed in conjunction with the fatty acid salt to make the latex. This is also taught in U.S. Pat. No. 3,062,767 where these materials act as latex stabilizers during and after the stripping operation. Nonionic stabilizers, since they cannot be coagulated with acids or multivalent cation salts, interfere with the ease of coacervation and can produce latices which are difficult to coacervate with acids or multivalent cation salts.
An additional technique for preparing latices is taught in U.S. Pat. No. 3,062,767 in which dilute aqueous emulsifier and raw latex are mixed and stripped of solvent. Use of excess emulsifier to stabilize emulsions is not desirable. Cook, Latex, Natural and Synthetic, Reinhold Publishing Corp., N.Y., 1956, page 51, states "in spite of the fact that they are very widely used there are at least two objections to the use of fatty acid soaps as latex stabilizers. They may increase viscosity more than desired and they may cause the latex to foam easily."
For many useful applications such as dipping, foam manufacturing and thread formation the desired latex stabilizer should give a stable raw latex and a mechanically stable finished latex without increasing viscosity and without interfering with coagulability.
The low unsaturation elastomer latices of the present invention offer the combined desirable properties of good mechanical stability, relatively low viscosity and excellent coagulability, the latter property being the ability of the finished latex to be cast or formed into films, threads and the like using chemical coagulants. Coagulability depends to a great extent on the use of an emulsifier system which will lose its surfactant properties upon addition of a chemical coagulant. Latices with good coagulability, such as those prepared in accordance with the present invention, will form films of about 0.02 inch thickness when treated with coagulant agents such as acids or calcium salts. Latices with poor coagulability characteristics will form films of only about 0.002 inch thickness.
Latices of the present invention also exhibit excellent mechanical stability having a coagulum value of about 1.5% by weight or less, based on the weight of raw latex, when subjected to high speed shearing or mixing. This property is evaluated by stirring in a high speed mixer at about 19,000 rpm for about 30 minutes at elevated temperature. The latices of the present invention usually exhibit a desirable percent coagulum of between about 0.5 and 1.2%.
The latices of the present invention also have a desirable low viscosity (Brookfield) in the range of about 1000 to 2000, preferably about 900 to 1200 centipoises. Viscosity, as is known in the art, is a function of many factors including particle size, solids content, type and amount of surfactant and the like. The composition of the present invention offers a viscosity in an optimum range and substantial improvements in viscosity are noted when latices prepared in accordance with the emulsifier system of the present invention are compared with latices produced using only a simple carboxylate salt emulsifier.
It has now been discovered in accordance with the present invention that low unsaturation elastomer latices, such as butyl rubber and EPDM latices, and in particular butyl rubber latices with a desirable low viscosity and excellent coagulability can be prepared using conventional equipment by employing a unique stabilizer salt derivative as part of the emulsifier composition at the time of emulsification. Improved mechanical stability in both raw and finished latex is achieved, without an undesirable increase in viscosity and without affecting ease of coagulation, through the use in the emulsifying agent of an additional ingredient which is a long chain aliphatic mono or dicarboxylic acid or anhydride in the form of its alkali metal, ammonium, or amine salt. The long chain aliphatic radical of the acid, diacid, or anhydride is generally derived from a lower alpha monoolefin which is polymerized or copolymerized by well known polymerization techniques to produce polymers of propylene, butene-1, or isobutylene having number average molecular weights ranging between about 500 and about 3500. One of the aforementioned mono-alpha olefins can be copolymerized with, for example, styrene, butadiene, isoprene, piperylene, chloroprene and the like to produce corresponding low molecular weight polymers. These materials upon condensation with unsaturated aliphatic carboxylic acids either mono- or diacids, or their anhydrides produce condensate acids, diacids, or anhydrides which are old in the art. Their method of production and their usefulness are shown in such patents as U.S. Pat. No. 3,172,892 and U.S. Pat. No. 3,448,048. In those patents, these compounds are produced and are useful, or derivatives thereof are useful, as dispersants in lubricating oil compositions. The unsaturated anhydride component with which the olefin polymers, or copolymers are condensed may be maleic anhydride or succinic anhydride. The free acids condensed with the olefin polymers or copolymers may be an unsaturated carboxylic acid such as acrylic acid, methacrylic acid, alpha methyl acrylic acid, crotonic acid, cinnamic, isocrotonic acid and the like. The instant invention does not reside in the formation of these condensation products but in their novel use in the form of an alkali metal, ammonium or amine salt. Examples 1 through 24 of U.S. Pat. No. 3,448,048 amply illustrate the method of formation of the acrylate condensate compounds and U.S. Pat. No. 3,172,892, example 1 shows how polyisobutenyl succinic anhydride is produced. Simple neutralization accomplishes the salt formation either in situ or as an external and separate neutralization operation. The molecular weight of the polymer reactant employed generally determines the molecular weight of the final condensates since the bulk of the molecular weight is represented by the polymeric or copolymeric substituent of the aliphatic carboxylic acid. Specifically, polyisobutenyl succinic anhydride in the form of its potassium or sodium salts having a number average molecular weight of from 850 to 1500 has been particularly useful. The polyisobutenyl acrylic acid salt in the form of its potassium or sodium salts and having a number average molecular weight of from about 850 to about 1500 is also useful from the standpoint of securing improved mechanical stability characteristics in butyl latices. The amount of the additive employed would generally run between about 0.5 and about 8.0 parts of the additive salt per 100 parts of butyl rubber contained in the latex, preferably from about 2 to about 6 phr of carboxylic acid salt is employed.
These amounts provide a two-component emulsifier composition consisting essentially of about 25% to 80% by weight of the aforesaid ammonium, alkali metal or amine salt of the long chain alpha olefin polymer or copolymer substituted organic unsaturated aliphatic mono or dicarboxylic acid or anhydride and 75% to 20% by weight of the ammonium or alkali metal C.sub.12 -C.sub.24 fatty acid soap. Preferably, these emulsifier compositions will contain approximately equal amounts of each component such as 45 to 55% of the polymeric component and 55 to 45% by weight of the C.sub.12 -C.sub.24 fatty acid soap component with optimum results being obtained when 50% by weight of each is employed. As a two-component emulsifier system the emulsifier composition of the present invention is believed to be a novel composition and as such constitutes a further embodiment of the present invention. Preferably the polymeric component is the potassium salt form of polyisobutenyl succinate or acrylate having a number average molecular weight of about 850 to 1500, such as about 1000 to 1300.
The resultant latex may be admixed with curing agents such as zinc oxide, sulfur, sulfide and thio accelerators and other conventional curing agents for use in producing dipped goods such as surgical gloves and so forth wherein the coagulation is accomplished by pretreatment of the articles being dipped with a strong mineral acid or certain organic acids such as formic acid or acetic acid, with inorganic metal salts of strong mineral acids such as calcium nitrate, calcium chloride, magnesium sulfate, or aluminum sulfate, or with isopropyl alcohol or other conventional coagulating agents, followed by curing the so coagulated material at a temperature of about 240.degree. F., for example for from 1 to 2 hours in an air oven, thus obtaining low unsaturation elastomer latex films of, for example, about 2400 psi tensile strengths and with elongations of, for example, about 1000%.